By preference, the ring should be below the plate, as this gives a greater
thickness of metal for drilling, and also makes it easy to jacket the tube
by sinking the plate into a wooden disc of somewhat greater diameter.
Under 50 lbs. of steam, a top of this kind attains a tremendous velocity.
Also, it flings the condensed steam about so indiscriminately that a ring
of zinc 3 inches high and 18 inches in diameter should be made wherewith to
surround it while it is running.
If a little bowl with edges turned over be accurately centred on the wheel,
a demonstration of the effects of centrifugal force may be made with water,
quicksilver, or shot, which fly up into the rim and disappear as the top
attains high speed, and come into sight again when its velocity decreases
to a certain figure. A perforated metal globe threaded on the spindle gives
the familiar humming sound.
A spectrum disc of the seven primary colours--violet, indigo, blue,
green, yellow, orange, red--revolved by the top, will appear more or less
white, the purity of which depends on the accuracy of the tints used.
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